1. Field of the Invention
The present invention relates to a tunnel valve sensor having a pinned layer structure with an iron oxide (Fe3O4) layer and, more particularly, to such a sensor wherein the highly polarized iron oxide (Fe3O4) layer improves the magnetoresistive coefficient dr/R of the spin valve sensor.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a tunnel junction sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes an insulative tunneling or barrier layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. The tunnel junction sensor is located between ferromagnetic first and second shield layers. First and second leads, which may be the first and second shield layers, are connected to the tunnel junction sensor for conducting a sense current therethrough. The sense current is conducted perpendicular to the major film planes (CPP) of the sensor as contrasted to a spin valve sensor where the sense current is conducted parallel to or in the major film planes (CIP) of the spin valve sensor. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is parallel to the ABS, occurs when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk.
When the magnetic moments of the pinned and free layers are parallel with respect to one another the resistance of the tunnel junction sensor to the sense current (IS) is at a minimum and when their magnetic moments are antiparallel the resistance of the tunnel junction sensor to the sense current (IS) is at a maximum. Changes in resistance of the tunnel junction sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When the sense current (IS) is conducted through the tunnel junction sensor resistance changes, due to signal fields from the rotating magnetic disk, cause potential changes that are detected and processed as playback signals. The sensitivity of the tunnel junction sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the tunnel junction sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the tunnel junction sensor at minimum resistance. The dr/R of a tunnel junction sensor can be on the order of 40% as compared to 10% for a spin valve sensor.
The first and second shield layers may engage the bottom and the top respectively of the tunnel junction sensor so that the first and second shield layers serve as leads for conducting the sense current IS through the tunnel junction sensor perpendicular to the major planes of the layers of the tunnel junction sensor. The tunnel junction sensor has first and second side surfaces which are normal to the ABS. First and second hard bias layers abut the first and second side surfaces respectively for longitudinally biasing the magnetic domains of the free layer. This longitudinal biasing maintains the magnetic moment of the free layer parallel to the ABS when the read head is in the quiescent condition.
The present invention increases the magnetoresistive coefficient dr/R of the tunnel sensor by providing the free layer structure with a cobalt iron (CoFe) layer and a half-metallic iron oxide (Fe3O4) layer, and providing the pinned layer structure with a cobalt iron (CoFe) layer and a half-metallic iron oxide (Fe3O4) layer. The half-metallic iron oxide (Fe3O4) layer in each of the free and pinned layer structures is highly polarized so that sense current electrons of only one spin are permitted to tunnel therethrough while sense current electrons of the opposite spin are prevented from tunneling. Accordingly, the magnetoresistance dr of the tunnel junction sensor, which is the resistance difference of a sensor between parallel and antiparallel relationships of the free and pinned layer structures, is significantly improved. The iron oxide (Fe3O4) highly discriminates between the spins of the sense current electrons as they tunnel therethrough.
The invention also employs cobalt iron (CoFe) in each of the pinned and free layer structures for different purposes. The cobalt iron (CoFe) layer employed in the pinned layer structure is located between the iron oxide (Fe3O4) layer and the pinning layer so that the oxide of the iron oxide (Fe3O4) layer does not degrade the microstructure of the pinning layer. Further, the cobalt iron (CoFe) layer in the pinned layer structure is Co50Fe50 which has a high positive magnetostriction. After constructing the head the positive magnetostriction causes a stress-induced anisotropy which supports the exchange coupling between the pinning layer and the cobalt iron (Co50Fe50) layer. The iron oxide (Fe3O4) layer in the pinned layer structure interfaces the barrier layer which is also an oxide layer and is compatible therewith. The cobalt iron (CoFe) layer of the free layer structure is preferably Co90Fe10 which has significantly lower magnetostriction and sufficient magnetic softness to counterbalance high coercivity of the iron oxide (Fe3O4) layer in the free layer structure. The iron oxide (Fe3O4) layer in the free layer structure also interfaces the barrier layer which is compatible therewith and is located between the barrier layer and the cobalt iron (Co90Fe10) layer. Additional information on spin polarized tunneling in ferromagnetic junctions can be found in the Journal of Magnetism and Magnetic Materials 200 (1999) 248-273 which is incorporated by reference herein.
An object of the present invention is to increase the magnetoresistance of a tunnel junction sensor.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.